1. Field of the Invention
This invention relates to a multiple channel (greater than two channels) birefringent multiplexer/demultiplexer. Accordingly, it is a general object of this invention to provide new and improved multiplexers and demultiplexers of such character.
2. General Background
U.S. Pat. No. 4,566,761 and the aforesaid pending application for "Birefringent Optical Multiplexer With Flattened Bandpass" present the advantages of birefringent wavelength division multiplexing (WDM) over previously used methods of wavelength division multiplexing, including dichroic mirrors, gratings, and holographic gratings methods. The problems inherent in multiple channel multiplexing/demultiplexing with a nonbirefringent WDM were stated in the cited U.S. Pat. No. 4,566,761.
The foregoing citations further describe a method of multiple channel multiplexing/demultiplexing with the birefringent WDM using a tree structure of filter stages. The four channel multiplexer/demultiplexer used three filter stages, A, B, C, where stage A had a channel separation of .DELTA..lambda. and stages B and C had channel separations of 2.DELTA..lambda.. The B and C stages were also required to be of lengths which were not equal but differed by d1=.lambda./(4.DELTA.n), where .DELTA.n is the birefringence of the crystal. Successive branches would require that the stages in the parallel branch all be of different lengths to an accuracy of 2 .mu.m. This stringent requirement would make for costly difficult production.